1. Field of the Invention
This invention relates to organosiloxane compounds. More particularly, this invention relates to organosiloxane compounds capable of functioning as inhibitors for compositions which cure by a platinum catalyzed hydrosilation reaction. The inhibitor extends the working time of these compositions without adversely affecting the cure rate of the compositions at temperatures of about 100.degree. C. and above.
Self-inhibiting polyorganosiloxane composition include one of the present compounds as at least a portion of the vinyl-containing polyorganosiloxane reactant in combination with an organohydrogensiloxane and a platinum hydrosilation catalyst.
2. Description of the Prior Art
One mechanism for curing organosiloxane compositions is by a hydrosilation reaction between a polyorganosiloxane containing two vinyl or other ethylenically unsaturated hydrocarbon radicals per molecule and an organohydrogensiloxane containing an average of at least three silicon bonded hydrogen atoms per molecule. Alternatively each molecule of the polyorganosiloxane contains three or more vinyl radicals and the organohydrogensiloxane contains two silicon bonded hydrogen atoms per molecule. The reaction is conducted in the presence of a platinum compound that is at least partially soluble in the reaction mixture.
In some instances the curing reaction will take place at temperatures of about 25.degree. C. once all of the reactants and catalyst have been combined. For some applications it is desirable to delay curing of the composition at these temperatures in order to store the composition for several hours prior to curing, to obtain additional working time or to ensure that the composition will completely fill all parts of a mold before curing to the extent that the composition is no longer flowable.
Many different classes of compounds will inhibit platinum catalyzed hydrosilation reactions. One type of inhibitor is volatile and must be removed from the composition by evaporation for the hydrosilation reaction to proceed. Nonvolatile inhibitors are inactivated by heating.
Volatile inhibitors include the vinyl-containing cyclic organosiloxanes disclosed in U.S. Pat. No. 3.923.705. which issued to Smith on Dec. 2. 1975. the acetylenic alcohols such as 2-methyl-3-butyn-3-ol described in U.S. Pat. No. 3,445,420 that issued to Kookootsedes et. al on May 20, 1969. and some of the heterocyclic amines such as pyridine disclosed in U.S. Pat. No. 3,188,299 that issued on June 8. 1965.
Non-volatile inhibitors are exemplified by the alkyl maleates disclosed in U.S. Pat. No. 4,256,870. which issued to Ekberg on Mar. 17, 1981, and the olefinic siloxanes described in U.S. Pat. No. 3,989,667, which issued to Lee and Marko on Nov. 2, 1976.
The olefinic siloxanes described in the aforementioned Lee and Marko patent are reaction products of secondary or tertiary acetylenic alcohols with siloxanes having silicon-bonded hydrogen atoms. The reaction products contain from 3 to 10 siloxane units with a total of at least three units of the formula RHSiO or R2HSiO.sub.0.5 and at least one unit of the formula ##STR1## where R represents a monovalent radical selected from the group consisting of hydrocarbon radicals and perfluoroalkylethylene radicals and R' represents a monovalent hydrocarbon radical substituted with a secondary or tertiary hydroxyl group.
Because the reaction products inhibit the hydrosilation reaction used to prepare them, the mixture of acetylenic alcohol, siloxane and platinum catalyst must be heated at temperatures above 100.degree. C. to inactivate the inhibitor. In accordance with the teaching of Lee and Marko, the mixture of reactants is passed through a tube wherein the mixture is heated above 100.degree. C. under sufficient pressure to maintain the reactants in the liquid state. The reaction product is typically a mixture wherein one or more of the silicon bonded hydrogen atoms on each molecule of the initial organohydrogensiloxane have been reacted.
The inhibitors of Lee and Marko are less than desirable because of the high temperatures and pressures required to prepare them and the difficulty of obtaining a pure material that will function reproducibly as an inhibitor.
Polydiorganosiloxanes containing vinyl radicals bonded to non-terminal carbon atoms are known materials. U.S. Pat. No. 3,498,945, which issued to Lefort et al on Mar. 3, 1970. discloses polydiorganosiloxanes containing at least one non-terminal vinyl radical and a viscosity of from 1 million to 25 million centipoise (1,000 to 25,000 Pa.s). The polydiorganosiloxanes are rubbery materials that are subsequently cured by a platinum-catalyzed reaction with an organohydrogensiloxane. Nowhere in the patent is there any indication that the disclosed vinyl-containing polymers can function as inhibitors to extend the working time of a curable composition at 25.degree. C.
U.S. Pat. No. 4,256,870, which issued to R. P. Eckberg on Mar. 17, 1981 and U.S. Pat. No. 4,347,346. referred to hereinafter as the '346 patent, which issued to the same inventor on Aug. 31, 1982. both disclose organosiloxane release coating compositions comprising a vinyl-containing polydiorganosiloxane, an organohydrogensiloxane, a platinum catalyst for promoting curing of the composition and an inhibitor for preventing premature curing of the composition. The vinyl-containing polydiorganosiloxane can contain vinyl radicals at both terminal and non-terminal positions, and can be represented by the formula ##STR2## where R is a monovalent hydrocarbon radical free of ethylenic unsaturation and R.sup.1 represents a vinyl or other ethylenically unsaturated hydrocarbon radical. and the value of x and y are such that the polymer contains up to about 20 weight percent of the vinyl or other ethylenically unsaturated hydrocarbon radical represented by R.sup.1. The '346 patent at column 9 lines 43 to 46 discloses that these polymers are produced by "well known means such as the KOH equilibration of methyltetramer (sic), methylvinyl tetramer and dimethylvinyl chainstoppers". Because no monovalent terminal units are specified one can only assume that the tetramers are cyclic diorganosiloxane oligomers.
The present inventor has found that polymerization of cclic diorganosiloxane oligomers using a basic catalyst such as potassium hydroxide results in multiple cleavages of silicon-oxygen bonds. When cyclic diorganosiloxane oligomers containing two different combinations of silicon-bonded hydrocarbon radicals, such as described in the examples of the '346 patent, are copolymerized the diorganosiloxane units from the cyclic methylvinylsiloxane are randomly distributed along the polymer molecule rather than being concentrated in groups as indicated by the structural formula on line 29 of column 5 of this patent.
U.S. Pat. No. 4.472,563, which issued to Chandra and Lo on September 18, 1984, discloses heat curable coating compositions comprising a polyorganosiloxane (A) containing at least one hydroxyl group or vinyl radical per molecule, an organohydrogensiloxane, a catalyst for promoting curing of the composition and an "ene-yne" compound of specified composition as a catalyst inhibitor. This patent teaches that the polyorganosiloxane A can be prepared by the hydrolysis and subsequent condensation of monomers containing a single silicon atom. It should be obvious to those skilled in the art of preparing organosiloxane copolymers that a reaction of this type involving two different monomers will yield polymers wherein the repeating units derived from the monomers are distributed randomly along the polymer molecule rather than being arranged in sequences as shown in the formulae on lines 11 through 15 of column 4 in this patent.